My Bangle JS2 battery has gone 37 days and is at 9% now

It should be possible to configure the GPS itself into a low power state, but the documentation
for the chip really isn't great

There was quite a bit of experimentation done a while back by @Mark_M (I think or Mark someone). I would post the thread but I never been that successful with the search tool in the forum.

One of the findings was that certain documentated features did not seem to respond and appear to be working. I have never had time to go back to it to look.

@Fteacher - might be worth testing your watch out on a window ledge where the GPS will get a fix. Download the AGPS data, switch on the run app and get a fix. Then leave it for 4 hours and check what the battery level is. Then repeat and add in the bluetooth heart monitor.